This invention relates generally to optical frequency multiplying devices and, more particularly, to frequency doubling devices known as Type II doublers. Frequency doubling devices take advantage of optical properties of some types of crystals, specifically a property relating to indices of refraction measured along different axes. The crystals used in doublers are anisotropic, meaning that their optical properties are not the same in all directions. The index of refraction is dependent both on the direction in which the index is measured and on the frequency or wavelength of the light. In a type I crystal, the index along one axis, as measured for a fundamental frequency of operation, matches the index along another axis, as measured for a second harmonic frequency. When a light beam polarized in one plane and at the fundamental frequency is input to the crystal at an appropriate angle, a second-harmonic output beam, polarized in an orthogonal plane, is produced. Typically, the angle of the crystal with respect to the input beam has to be adjusted to tune or phase-match the device to provide a near-exact index match to the second harmonic output beam.
Type II doublers operate on a slightly different principle. An input beam is split by the crystal into two orthogonal components which are, in effect, frequency summed in the crystal to produce a double frequency output beam. Type II doublers are generally preferred because they are more efficient. The Type II crystal may also need to be tilted to tune the device to provide the desired output. An important difference between the two types of doublers is that Type I doublers do not change the polarization of the input beam, but a simple Type II doubler acts as a random waveplate and converts the input beam into an elliptically polarized beam. Moreover, tilting the crystal for any reason, such as to optimize phase matching, will change the ellipticity of the beam polarization.
An important application of doublers is in phase conjugated master oscillator power amplifiers (PC MOPAs), to provide visible light output from such devices. Phase conjugated frequency doubling requires that the doubler be placed within the PC MOPA, and aberrations caused by the doubler are then canceled in a second pass through the doubler, after phase conjugation. Type I doublers have been used in this type of configuration. Type II doublers, however, which inherently introduce some birefringence (as much as 50% for circular polarization), are not ideally suited to PC MOPA applications.
Accordingly, there is a need for a Type II frequency doubler structure that can be used in a PC MOPA or in a laser resonator without disturbing the polarization state of the laser beam. In other words, there is a need for a Type II frequency doubler that is birefringence-compensated. The present invention is directed to this end.